The present invention relates to nuclear process off-gas treatment systems, and more particularly to a method for selectively removing and recovering for longterm storage, if necessary, the noble gas and other gaseous components typically emitted during nuclear process operations. The described method is adaptable and useful, for example, in treating the dissolver off-gas effluents released during reprocessing of spent nuclear fuels whereby to permit radioactive contaminant recovery prior to releasing the remaining off-gases to the atmosphere.
Activation and fission products are released and become airborne during nuclear fuel reprocessing of light water reactor fuels. Certain of these products are long-lived and, accordingly, represent a serious hazard to people if the products are released directly into the local environment. The products of primary concern are tritium, carbon-14, krypton-85, iodine-129, and some of the fission product semivolatile metal oxides such as ruthenium, technetium, and antimony. Iodine-131 may also be present in hazardous amounts if the spent nuclear fuel is reprocessed within about 180 days following reactor discharge. In most instances the radioxenons will have decayed prior to fuel reprocessing.
Various methods have been used and proposed which influence the distribution of airborne radioactive contaminants carried in the reprocessing off-gas stream. Some methods propose a preliminary heat-treatment step following shearing of the spent fuel bundles whereby to release a large fraction of the volatile activation and fission product gases. Most of the remaining volatile gases are subsequently released during the fuel dissolution step. In the methods employing a heat-treatment step three off-gas streams containing activation and fission product gases are released: (1) an off-gas stream from the heat-treatment step; (2) an off-gas stream released from the dissolver; and (3) an off-gas stream from the various vessels comprising the remainder of the fuel reprocessing system.
Where a heat treatment step is included in the design of an off-gas treatment system, essentially all of the tritium and appreciable quantities of the iodine and noble gases will be released to the heat-treatment step off-gas stream. Due to the high concentration of tritium in this off-gas stream, the most practical approach is to remove the tritium, and possibly the iodine, and then return the noble gases and other carbon-containing gases that may be present to the off-gas stream released from the dissolver. If a heat-treatment step is not included in the method, the tritium will be released during dissolution of the fuel and most of the tritium will exchange with the nitric acid dissolver solution. Noble gases present in the dissolver will be quantitatively released to the dissolver off-gas stream. Most of the carbon-14 containing compounds, the majority of which are in the form of carbon dioxide, and greater than 95% of the iodine will also be released to the dissolver off-gas stream.
Those activation and fission product species not released during dissolution will follow the aqueous phase of the solvent extraction system and will be released to the various vessel vents which make up the vessel off-gas stream. Some iodine admixed with these species may eventually reach the high-level liquid waste tanks.
Two main processes have been proposed or used for recovering noble gases, and in particular noble gases present in the dissolver off-gas stream: cryogenic distillation and flourocarbon adsorption. These processes are reviewed and discussed in U.S. ERDA "Airborne Waste Recovery and Immobilization" Alternatives for Managing Wastes from Reactors and Post-Fission Operations in the LWR Fuel Cycle, ERDA-76/43, Volume II, May 1976.
There are several versions of the cryogenic distillation method that differ primarily in their approach to removing contaminant gases, other than xenon and krypton, in a nitrogen carrier gas stream. These processes have been described at several recent conferences and in publications promulgated to the various participants. IAEA, Proceedings of International Symposium on the Management of Radioactive Wastes from the Nuclear Fuel Recycle, Vienna, Austria, 22-26 March 1976; and M. W. First, Ed., Proceedings of the 15th DOE Nuclear Air Cleaning Conference, 7-10 August 1978, Boston, Mass., CONF 780819, February 1979.
To prevent the freeze-out and plugging of contaminant gases in a cryogenic distillation unit, an efficient pre-treatment contaminant gas-clean-up must be used. Generally, contaminant gases, such as CO.sub.2, NO.sub.x (including N.sub.2 O, NO, NO.sub.2, and N.sub.2 O.sub.4), and various hydrocarbons, must be removed to about 1 ppm. to ensure trouble-free operation. Because of the potential hazard of ozone, a radiolysis product of oxygen in high radiation fields caused by concentrated krypton-85, the oxygen concentration in the off-gas stream should be kept low. J. F. Riley, "Radiolysis of Liquid Oxygen and Oxygen-Nitrogen at 77.degree. K.," Chemistry Division Annual Progress Report for Period Ending June 20, 1963, ORNL-3488.
The fluorocarbon adsorption process for noble gas separation from reprocessing plant off-gas effluents is less developed. Although pilot-scale testing has been performed, a complete description, including the disposition of all the collected products and the expected system decontamination factors (DFs), has not yet appeared.
Yet another method, cryogenic selective adsorption-desorption, has been proposed for the removal of noble gases. T. Kanazawa, et al., "Development of the Cryogenic Selective Adsorption-Desorption Process on Removal of Radioactive Noble Gases," Proceedings of the 14th ERDA Air Cleaning Conference, 2-4 August, 1976, Sun Valley, Idaho, CONF-760822, February 1977.